Magnetic fluid drive units are known that comprise a heating section and a magnetic field applicator in a flow path encapsulating a magnetic fluid (PTLs 1 and 2).
In magnetic fluid driven technology, a magnetic field is applied to magnetize a magnetic fluid, and a portion of the magnetized magnetic fluid is heated to lower the magnetization of the heated magnetic fluid. This produces a disproportion in the magnetic volume force acting on the magnetic fluid, allowing the magnetic fluid to be driven. A magnetic fluid typically contains magnetic fine particles such as iron oxide fine particles and a mother liquor in which the magnetic fine particles are dispersed, and if desired the magnetic fluid may further contain a low-boiling-point solvent having a lower boiling point than that of the mother liquor.
An example of a publicly known magnetic fluid drive unit is shown in FIG. 1.
In the magnetic fluid drive unit of FIG. 1(a), with the x position on the magnetic fluid flow path as the abscissa and magnetic field H as the ordinate, the magnetic field applicator ideally generates a magnetic field exhibiting an approximately trapezoid distribution with no polar inversion (FIG. 1(b)). When a magnetic field H is applied to a magnetic fluid, the magnetic fluid behaves as a fluid with magnetization M. Iron oxide fine particles behave in a super-paramagnetic manner at room temperature. Magnetization of a super-paramagnetic body obeys the Langevin function, and for low magnetic field ranges the magnetization can be approximated as being proportional to the magnetic field. The Curie temperature for iron oxide fine particles is 477K (204° C.), and their temperature-sensitive property is such that the magnetization falls with increasing temperature toward the Curie temperature T.
Thus, local magnetization M of a magnetic fluid is expressed by the following mathematical formula.
                    M        =                              μ            0                    ⁢                      χ            ⁡                          (                              1                -                α                            )                                ⁢                      (                          1              -                                                T                  -                                      T                    0                                                                                        T                    c                                    -                                      T                    0                                                                        )                    ⁢          H                                    [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            
The symbols in the formula represent the following.
μ0: Vacuum permeability
χ: Magnetic susceptibility
α: Void percentage of magnetic fluid
T: Temperature of magnetic fluid at heating section
T0: Temperature of magnetic fluid at non-heating section
Tc: Curie temperature of magnetic fine particles
H: Magnetic field
In a magnetic fluid under a magnetic field H, a magnetic volume force F acts in direct proportion to the magnetization M and magnetic field gradient ∇H (F=M·∇H). The magnetic volume force F undergoes a sign reversal bordering on the center of the magnetic field application site, when the position x on the magnetic fluid flow path is plotted on the abscissa and magnetic volume force F is plotted on the ordinate (FIG. 1(c)). The total driving force acting on a magnetic fluid is proportional to the volume of the area bounded by the curve of the magnetic volume force F and the abscissa x, as shown in FIG. 1(c).
At the stage prior to heating, balance between the magnetic volume force F1 in the right direction and the magnetic volume force F2 in the left direction in FIG. 1 prevents driving of the magnetic fluid (“(i) Before heating” in FIG. 1(c)).
When a portion of a magnetized magnetic fluid is heated by a heating section situated at one end of the magnetic field applicator in the magnetic fluid flow path, magnetization of the iron oxide particles at the heating section decreases as the temperature T increases, resulting in reduced magnetization M of the magnetic fluid. As a result, the magnetic volume force F2 of the heating section is smaller than the magnetic volume force F1 at the non-heating section, and consequently a driving force is produced in the right direction in FIG. 1, as the difference between F1 and F2. The magnetic fluid thus begins to be spontaneously driven in the right direction in FIG. 1 (“(ii) During heating (T<TL)” in FIG. 1(c)).
When the magnetic fluid contains the aforementioned low-boiling-point solvent, and the magnetic fluid is heated to a temperature of at least the boiling point TL of the low-boiling-point solvent and below the boiling point TH of the mother liquor, the low-boiling-point solvent gasifies, generating air bubbles inside the magnetic fluid. This also causes the void percentage a of the magnetic fluid to increase, further reducing the magnetization M of the heating section. Thus, the difference between the magnetic volume force F1 in the right direction in FIG. 1 and the magnetic volume force F2 in the left direction in FIG. 1 increases further, such that the total driving force in the right direction in FIG. 1 increases (“(iii) During heating (T≤TL<TH)” in FIG. 1(c)).
PTL 1 relates to technology in which the heating zone of the heating section in the magnetic fluid flow path is controlled to arbitrarily vary the driving direction and/or driving speed of the magnetic fluid, and/or the heat volume at the heating section in the magnetic fluid flow path is controlled to arbitrarily vary the driving speed of the magnetic fluid.
PTL 2 relates to technology using a permanent magnet to apply a magnetic field at a magnetic field applicator in a magnetic fluid flow path.